The $concat operator joins multiple string values into one string in MongoDB aggregation pipelines. Use it to build full names, formatted labels, addresses, and display-friendly output from separate fields.
Combine fields + literals.
Array of expressions.
Add spaces, commas, etc.
Create display fields.
Names, reports, labels.
Guard with $ifNull.
In MongoDB’s aggregation framework, the $concat operator concatenates an array of string expressions into a single string. For example, { $concat: ["$firstName", " ", "$lastName"] } produces "John Doe" when firstName is John and lastName is Doe.
Include literal separators like " " (space), ", " (comma-space), or "-" between field references. Without them, values are joined with no gap.
The $concat operator takes an array of string expressions:
{ $concat: [ <expression1>, <expression2>, ... ] }$concat — returns one string built from all array elements."$firstName"), literals (" "), or nested string expressions.$project, $addFields, or $set.null, the entire result is null.$toString first.If any part of the concatenation is null, the whole result becomes null:
$concat: ["John", " ", "Doe"] → "John Doe"$concat: ["John", " ", null] → null{ $ifNull: ["$lastName", ""] } per field| Question | Answer |
|---|---|
| Operator type | Aggregation expression operator (string) |
| Syntax | { $concat: [ expr1, expr2, ... ] } |
| Input types | Strings (use $toString for numbers) |
| Null behavior | Any null → entire result is null |
| Common stages | $project, $addFields, $set |
{
$concat: ["$first", " ", "$last"]
}Join with space
{
$concat: ["Order #", "$id"]
}Prefix + field
$ifNull per fieldPrevent null output
$concatArraysJoins arrays, not strings
Build full names, formatted addresses, and null-safe labels using $concat in aggregation pipelines.
Use a students collection and build a fullName field with $project.
Suppose you have a students collection with separate name fields:
[
{ "_id": ObjectId("609c26812e9274a86871bc6a"), "firstName": "John", "lastName": "Doe", "grade": "A" },
{ "_id": ObjectId("609c26812e9274a86871bc6b"), "firstName": "Alice", "lastName": "Smith", "grade": "B" },
{ "_id": ObjectId("609c26812e9274a86871bc6c"), "firstName": "Michael", "lastName": "Johnson", "grade": "C" }
]Concatenate firstName and lastName with a space separator:
db.students.aggregate([
{
$project: {
fullName: { $concat: [ "$firstName", " ", "$lastName" ] },
grade: 1
}
}
])MongoDB evaluates each array element left to right and joins them into one string. The literal " " inserts a space between first and last names.
Handle missing values, format addresses, and combine numbers with strings.
Prevent a missing lastName from nullifying the entire result:
db.students.aggregate([
{
$project: {
fullName: {
$concat: [
{ $ifNull: [ "$firstName", "" ] },
" ",
{ $ifNull: [ "$lastName", "" ] }
]
},
grade: 1
}
}
])
// firstName: "John", lastName: null → fullName: "John "
// firstName: null, lastName: "Doe" → fullName: " Doe"$ifNull replaces missing fields with an empty string so $concat can still produce output. Trim with $trim if trailing spaces matter.
Join multiple address parts with commas for report output:
db.customers.aggregate([
{
$project: {
name: 1,
fullAddress: {
$concat: [
{ $ifNull: [ "$street", "" ] },
", ",
{ $ifNull: [ "$city", "" ] },
", ",
{ $ifNull: [ "$state", "" ] },
" ",
{ $ifNull: [ "$zip", "" ] }
]
}
}
}
])
// → "123 Main St, Springfield, IL 62701"Mix field references and literal separators freely. This pattern is common for CSV exports, mailing labels, and dashboard display fields.
$concat only accepts strings. Convert numbers before joining:
db.orders.aggregate([
{
$project: {
orderLabel: {
$concat: [
"Order #",
{ $toString: "$orderId" },
" — $",
{ $toString: "$total" }
]
}
}
}
])
// orderId: 1042, total: 59.99 → "Order #1042 — $59.99"Wrap numeric fields with $toString before passing them to $concat. For formatted currency, consider $round before converting.
$concat WorksField references and literals in the array are resolved to string values (or null).
Values are appended left to right with no automatic separator. If any value is null, the result is null.
The result is written to the field you define in $project or $addFields.
Ready for display, export, or further string operations.
The $concat operator is an essential string tool in MongoDB aggregation pipelines. It lets you merge fields and literals into a single readable value — perfect for names, addresses, order labels, and report formatting.
For beginners, remember: include separators explicitly, guard null fields with $ifNull, and use $toString for numbers. For joining arrays, use $concatArrays instead.
$ifNull$toString before concatenating numbers$trim after concat if spaces may trail$concat with $concatArrays$toString$concatUse these points when joining strings in MongoDB.
Array of expressions.
Purpose{ $concat: [a, b] }
Literals like " " or ", ".
TipUse $ifNull to guard.
Edge caseConvert with $toString.
TypesMove on to $concatArrays for merging arrays, or review $arrayToObject for data transforms.
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